US2039800A

US2039800A - Silencer

Info

Publication number: US2039800A
Application number: US681093A
Authority: US
Inventors: William A Jack
Original assignee: CF Burgess Laboratories Inc
Current assignee: CF Burgess Laboratories Inc
Priority date: 1933-07-19
Filing date: 1933-07-19
Publication date: 1936-05-05
Anticipated expiration: 1953-05-05

Description

May 5, 1936.

w. A. JACK, 30 2,039,800

SILENCER Filed July 19, 1933 I 3 Sheets-Sheet l ATTORN EYS May 5, 1936. w. A. JACK, an

S ILENCER Filed July 19, 1933' 3 Sheets-Sheet 2 INVENTOR Wm. fl. few/r 3rd,

ATTORNEYS Patented May s, 1936 UNITED STATES SILENCEB William A. Jack, 311, Madison, Win, asslgnor to C. F. Burgess Laboratories, Inc., Madison, Wis a corporation of Delaware Application July 19, 1933, Serial No. 881,093

23 Claims. (01. 181-42) This invention relates to silencers of the type used in connection with intakes and exhausts of gas engines and air compressor intakes, on blowers, and for noise producing moving gases from other sources.

This invention is an improvement on the socalled resonator type of silencer in which one or more tuned resonators are used to attenuate sounds passing through a duct. It is an object of this invention to provide a novel form of sound attenuating resonator in which the three components of the acoustic impedance thereof are structurally substantially separate so that each component may be changed independently 7 of the others whereby for any given volume of resonance chamber the resonance frequency may be changed over a wide frequency band without aifecting materially the decrement (sharpness of tuning) and, also, the decrement may be varied substantially independently of the resonance frequency to produce the desired characteristics. It is a. further object to provide a resonator of such construction that a single chamber efllciently attenuates a broad band of frequencies. The acoustical terms used herein are those approved by the Acoustical Society of America.

In the accompanying drawings, various forms of this invention are illustrated in diagrammatic form. This invention, however, is not limited to these specific forms.

Fig. 1 is a sectional view of a simple type of silencer utilizing this invention;

Fig. 2 is a cross sectional view of Fig. 1 on line 2-4;

Fig. 3 is a sectional view of a silencer with a double length entrance chamber and a high frequency silencer;

Fig. 4 is a cross sectional view on line 4-4 of Fig. 3;

Fig. 5 is a sectional view of a silencer incorporating my invention with a second resonance chamber in series with the primary chamber, and disclosing the use of sound absorbing material also;

Fig. 6 is a similar view of a silencer with compound resonance chambers of my invention and a, high frequency silencer;

Fig. 7 is a similar view of a silencer containing two resonance chambers of my invention in parallel;

Fig. 8 is a similar view of another form of silencer incorporating my invention;

Figs. 9 and 9a. are cross sectional views of a different type of the silencer of my invention containing two resonance chambers;

Fig. 10 is a. cross-sectional end view of another type of the silencer of my invention containing two resonance chambers;

Fig. 11 is a. cross sectional end view of a silencer which is not circular in cross section incorporating my invention;

Figs. 12 and 13 are sectional views of modified forms of silencers incorporating my invention in which a central resonance chamber is incorpo rated;

Fig. 14 is a sectional view of one form of exhaust silencer incorporating this invention;

Fig. 15 is a similar view of another type of resonator construction in which the principles of this invention are incorporated; and

Fig. 16 is a similar view of another modification;

Fig. 17 is a cross sectional view along the radial entrance of a square silencer having an ofiset coupled to the annular radial passage I4 formed by spacing an annular disk 85 interiorly of and adjacent one ofthe end flanges I6 and I1. Openings i3 and annular radial passage I4 constitute the neck of the resonator. Radial passage I4 preferably is substantially of the same width throughout its length and opens into the duct ll, preferably without any restriction at this entrance. Any restrictionor restrictions in the radial passage add to the resistance. In this construction (within limits) the resonance chamber l2 forms substantially the entire compliance (capacitance) of the resonator, the openings I3 form substantially the entire resistance and the radial passage l4 forms substantially the entire inertance (inductance) of the resonator, that is, the construction is such that the three components of the acoustic impedance of the resonator are structurally substantially separate. For example, the resistance of radial passage I4 is sistance of openings II. In extreme cases there is more overlapping, as, for example, when the radial passage I4 is very narrow and the resistance thereof becomes comparatively large.

In the constructions of this invention in which the three components of the impedance of the resonator are structurally separate, the designing of a resonator for attenuating undesired frequencies is greatly simplified. Furthermore it is possible structurally to locate the three components so that each component and the entire combination may function at high efliciency. The radial type neck offers a minimum of resistance to the oscillatory movement of the sound waves entering it. The improved results obtained are evidenced by the excellent attenuation it is.

possible to obtain with a single chamber in any single silencer over a broad band of frequencies and especially broad bands of low frequencies when a compliance chamber of small volume must be used.

In a specific intake silencer for an automobile the duct II was 1% inches in diameter, the resonance chamber had an outside diameter of 7% inches and was 3 inches long. The radial passage was inch wide and there were about 45 holes inch in diameter spaced on about inch centers at about inch from the outer periphery of disk I 5. This silencer attenuated satisfactorily a continuous band of frequencies of from cycles to about 400 cycles, though peaking at 200 cycles. These data are illustrative of the wide band of frequencies, and especially low frequencies, which may be attenuated satisfactorily by a small single resonance chamber with a neck made in accordance with the principles of this invention. It is especially desirable that automobile intake silencers attenuate a wide band of frequencies because of the wide range of engine speeds used in the operation of automobiles.

, The construction has several advantages over the structures of the prior art. It is possible to vary the resonance frequency of the resonator over a considerable range by changing the length and/or width of the radial passage it (within limits) without affecting the decrement substantially. However, if the width of the radial passage is decreased greatly as for example, to less than inch in the above specific intake silencer, the resistance thereof increases rapidly and is no longer negligible compared to the resistance of the openings II. Such narrow widths are seldom used because the attenuation is decreased greatly by the high resistance. Decreasing the width or increasing the length of the passage lowera the resonance frequency. Fig. 1 shows a construction wherein the width of the inertance passage may be varied so that the silencer may be readily tuned to any frequency. End flange I6 is provided with an inwardly extending interiorly threaded shoulder 9 adapted to engage screw threads upon the exterior of the shoulder 8, as shown. By adjusting the position of the end flange toward and away from the annular disc IS, the width of inertance passage l4 may be varied. The spaced openings I3 preferably are placed at points farthest removed from the duct ll so that advantage may be taken of the full length of the neck or radial passage I. These openings need not be placed immediately adjacent the outer edge of the disk l5 so far as the effect of the inertance on the resonance frequency is concerned. However, if they are placed substantially neglig ble as compared to the re-,,

too far from the outer edge the inertance of the passage is affected.

The decrement of the resonator is dependent upon the aggregate area of openings 13 and this area may be regulated to obtain the desired results. If the total area is large, the resistance is low and the decrement is small, that is the "tuning" is sharp, the attenuation at the peak frequency being more pronounced. As the area of opening is decreased the sharpness of tuning and the peak attenuation decreases, while the attenuation at the adjacent frequencies increases. The tuning thereby is broadened so that a wide band of frequencies is attenuated efiiciently. If the total area of opening is decreased too much the attenuation also is decreased and the silencing is impaired. It is desirable to have the holes spaced closely enough to each other so that no portion of the pressure waves will be blocked, which would decrease the attenuation. A circular narrow slot may be used but the spaced holes are preferred for commercial production. If the disk I 5 is a thin sheet, the holes have practically no inertance or compliance in comparison with the balance of the resonator. By changing the size and/or number of holes it becomes possible (within limits) to change the resistance or decrement without appreciably affecting the resonance frequency. This may be accomplished in the silencer shown in Fig. 1 by removing the flange IS. The holes l3 may then be increased in diameter or number. In practice it is desirable to start with a small number of holes or holes of a small diameter and increase the size and/or number until the desired decrement is obtained. However, excess holes may be closed by lead, putty or other like suitable material. This type of construction provides a concentrated resistance between the inertance and compliance instead of a resistance distributed throughout the neck as in attenuating resonators of the prior art. Although the concentrated resistance may be placed at any point in the neck the most desirable place is between the main compliance and the main inertance. If shoulders or flanges are formed at the edge of openings l3 the inertance of these openings is increased and therefore such shoulders or flanges should be omitted or made small if the inertance of the neck of the resonator is to be independent of the area of the resistance openings.

In the practical application of this invention the advantages are apparentin the following examples. In a resonator whose resonance frequency is too high and which has a resonance chamber of a given volume which can not be enlarged because of space limitations, it is possible to decrease the resonance frequency without appreciably .disturbing the decrement or appreciably affecting the attenuation by merely increasing the length of the radial passage if the width can not be decreased because of decreased attenuation which would result. The dimensional changes occasioned thereby are small as compared to the change in the compliance volume which would be necessary for the same frequency change. The length may be increased by the construction shown for radial passage 2 I, 22 in Fig. 3. The decrement is not disturbed appreciably because the change in length of the radial passage does not affect appreciably the total resistance of the resonator. Any slight decrement change which may result may be corrected by changing the area of openings l3 slightly. In the constructions of the prior art an increase in length or the neck oi the resonator not only aflects thr Figs. 9 and 9a are cross-sections of a modiinertance but it also affects the resistance vsubstantially and thereby changes the decrement. 0n the other hand, in the constructions of this invention, it the volume of the resonance chamber can not be changed and it is necessary to change the decrement this maybe eflected without aiiecting the resonance frequency by merely changing the total area or the openings i3. The advantages of this construction are apparent to those skilled in the art.

In Fig. 3 the sound duct I 8 is surrounded in part at least by an annular resonance chamber I 9. The entrance passage to this chamber consists of radial passages 2| and 22 connected with each other in series by annular opening 23, and concentrated resistance openings 24 in annular disk 25. Annular opening 23 should be of an area which will not cause any appreciable increase in the resistance of the radial passages 2| and 22. Although this combination of the two radial passages is not as eflective as a single radial passage whose length is equal to the combined length of the two radial passages, nevertheless this construction is more compact and may be used where space requirements prohibit the use of .a long Fig. 5 illustrates a compound resonator. In additon sound absorbing material is used to silence high frequencies in the sound passing through the duct 28. As in Fig. 1 the resonance chamber 29 is connected with the duct by means of the radial passage 38 and the concentrated resistance openings 3|. A second resonance chamber 32 is formed adjacent the resonance chamber 29 by means of annular disk 33, the two chambers being connected by suitable openings 34. Such a compound construction may be used for special purposes. The sound duct 28 is also lined by sound absorbing material 35 faced by perforated metal 38 or other foraminous material or other retaining material. The perforated or other foraminous facing may be omitted when certain types of molded absorbing materials are used.

In Fig. 6 duct 31 is immediately surrounded by small annular chamber 38 for the removal of high frequencies, this chamber in turn being surrounded by the compounded resonance chambers 39 and 48. The neck of chamber 39 is formed by radial passage 4| opening into duct 31 and concentrated resistance openings 42. Connection between chambers 39 and 48 is made by means of radial passage 43 and concentrated resistance openings 44.

In Fig. 'l the duct 45 is surrounded by two an

nular resonance chambers

48 and 41. The inner chamber 46 connects with duct 45 by means oi radial passage 48 and concentrated resistance openings 49. The outer resonance chamber 41 connects with duct 45 through radial passage 50 and concentrated resistance openings 5|.

In Fig. 8 the silencer consists of a short cylinder of large diameter. In this construction the sound duct 52 is connected wtih the resonance chamber 53 by a separately inserted radial passage or slot 54 and concentrated resistance openings 55. The openings may also be in the periphery 5B.

flcation or the silencer through the radial passage I4 01' Fig. 1. The duct 81 is surrounded by two

segmental resonance chambers

58 and 59 formed by segmental partitions and iii inserted in the resonance chamber. These segmental partitions extend through the radial passage and form two segmental radial passages. These segmental radial es open into duct I1 and are connected with the segmental chambers by concentrated resistance openings 62.

Fig. 10 is a cross sectional view through the radial passage 01 a silencer somewhat similar to that 01' Fig. 7 and having two

annular resonance chambers

93 and 84 which surround the duct 65. The resonance chambers are reached through a common radialpassage and are separated from each other by an annular partition 86 which does not extend into this passage.

Concentrated resistance openings

81 and 89 are formed between the common radial passage and chambers 83 and 64 respectively. If chamber 64 is of relatively small volume it attenuates a range of frequencies above the range attenuated by chamber 63.

In Fig. 11 is shown the cross sectional end through the radial passage of a silencer in which the cross section is not round. This oval form may be applied to any 01' the types previously described. The duct 69 is surrounded by the resonance chamber 18 reached by a radial passage as in Fig. 1 and concentrated resistance openings 1 l The contour of the silencer, however, may be any desired shape such as square, hexagonal, etc., and the duct may be oiI -center as illustrated in Figs. 17 and 18.

In Fig. 12 the sound duct 12 surrounds the resonance chamber 13. A disk I4 is spa'ced from the end wall 15 of the chamber and forms a radial passage 16. This passage communicates with the interior of the chamber 13 by means of concentrated resistance openings 11. One opening may be used instead of the several shown. The front of disk 14 may be faced with sound absorbing material 14a. An air cleaning material I8 may be inserted in duct 12 if the silencer is to be used for an automobile engine intake.

In Fig. 13 the construction is similar to Fig. 12 except that the radial passage I9 between the duct 19m and concentrated resistance openings 88 in the end wall 8| of the resonance chamber 82 is formed in another way. The radial passage I9 formed by spacing disk 83 away from end wall 8| is closed at its periphery. Entrance is made from duct 190. by central opening 84.

Fig. 14 shows this invention applied to one type.

01' exhaust silencer. Because of the intensity of the pressure waves which have to be silenced it is desirable to use a series of

chambers

85, 86 and 81 which are connected to the main duct 88 by means of

radial passages

89, 99 and 9| and

concentrated resistance openings

92, 93 and 94, respectively.- Because 01' the hard service to which exhaust muillers are subject it is not desirable to have any portion of the duct 88 floating. In Fig. 14 this duct is shown as made in one piece.

Large openings

95, 96 and 91 in duct 88 provide the connections with

radial passages

89, 98 and 9|, respectively. The resistance at these entrances is thereby kept' at a minimum and the structure is oi. suflicient strength for exhaust muiiier purposes.

Although Figs. 1 to 14 show the principle of concentrated impedance components applied specifically to resonators in which a radial entrance is used this principle may be applied to resonators all of the prior art. For example, Figs. 15 and 16 show a silencer in which the neck of the resonator is of the annular type entrance of the prior art, and the concentrated resistance between the inertance and compliance is shown at two different points. In Fig. 15 duct is surrounded by inner and outer annular shells 00 and I00, respectively. The compliance chamber II is acoustically connected with duct 00 by means of the annular inertance passage I02 between the wall of duct 90 and shell 00, the opening I03 in the duct wall and concentrated resistance openings I04 in the end flange I00 between the shell 09 and the wall of duct 90.

In the construction shown in Fig. 16 the duct 00 is surrounded by inner and outer annular shells I06 and I0'I, respectively. The compliance chamber I00 is acoustically connected with duct 00 by means of the annular inertance passage I00 between the wall of duct 00 and shell I00, the opening H0 in the duct wall and concentrated resistance openings III in annular shell I06 at the end removed from opening H0. Flange II2 closes the end of the annular passage. The two constructions of Figs. 15 and 16 may be combined, that is. concentrated resistance openings may be used in both the inner annular shell and the end flange in the same resonator.

If the principle of concentrated impedance components is applied to the resonators of the prior art as in Fig. 15 it is desirable to design the structure supplying the inertance component so that it has a small or substantially negligible resistance as compared to the concentrated resistance, so that the maximum benefit may be obtained.

In Figs. 1 to 14 the radial necks are shown to be of constant width. Under certain conditions it may be desirable to change the width of the radial passage along its length. For example, in Fig. 19, the disk I is replaced by a disk I I0 in the form of a flat truncated cone to thereby provide a radial passage I which varies in width throughout its length. Resistance openings H5 lead from the radial passage to the compliance chamber II6 enclosed in shell I. The radial passage opens into duct III in which occur the undesirable sounds which are attenuated. In Fig. 20 is shown the same modification applied to the annular neck of Figs. and 16. The annular neck of these modifications is changed in Fig. to the shape of truncated cone H0 containing resistance openings I20 which lead into compliance chamber I2I within shell I22. The neck opens into duct I23 in which occur the sounds to be silenced. Other variations will be apparent to those skilled in the art.

Sound frequencies which are attenuated vary in some inverse function of the volume of the resonance chambers. While the resonators illustrated herein incorporating the herein described invention are mostly of the low frequency absorbing type the same principle is used in resonators that attenuate high frequencies. For example in Fig. 19 the conventional high frequency absorbing chamber 26 of Fig. 3 is replaced by a chamber I24 of about equal size with a radial entrance I25 and concentrated resistance openings I26. This construction may be used to attenuate a narrow or wide band of high frequencies depending upon the total area of the resistance openings. The high frequency attenuating chamber of Fig. 6 may be modified similarly as shown in Fig. 19 so that the neck I21 of the resonance chamber I20 is radial with high resistance openings I20 between the chamber and the radial neck.

It may also be advantageous under certain conditions to introduce a sound attenuating material such as felt, mineral wool, metallic wool, pumice granules and the like in the resonance chambers. This material may be especially helpful in the small high frequency attenuating chambers. Such a construction is shown in Fig. 19 in which high frequency attenuating chamber I30 having radial neck Ill and resistance openings I32 is filled with cotton linters I03.

I claim:

1. In combination with a duct, a silencer comprising a shell having flanges attached thereto surrounding at least a portion of said duct and enclosing an annular chamber, an annular disk spaced a uniform distance from and adjacent interiorly of one of said flanges and forming an end wall of said chamber, the space between said flange and said disk opening into said duct, said annular disk having one or more acoustic-resistance openings therein.

2. In combination with a duct, a silencer comprising an annular chamber divided into two or more segmental parts surrounding at least a portion of said duct, segmental radial acoustic inertance passages registering end-wise with the segmental portions of said chamber and of substantially negligible acoustic resistance opening into said duct, and an acoustical resistance opening or openings between the corresponding segmental radial passages and segmental chambers.

3. In combination with a duct, a silencer comprising a resonator which comprises a chamber associated with said duct, an inertance passage opening into said duct and being of such shape and dImenslons as to provide substantially all of the inertance of said resonator and having substantially negligible acoustic resistance, and a concentrated acoustic resistance of substantially negligible inertance comprising one or more openings between said chamber and said inertance passage to thereby form a construction in which said passage and said openings each respectively provide substantially all the inertance and resistance of the neck of said resonator.

4. In a silencer in which objectionable sounds are attenuated by means of a tuned resonator, the combination of a resonance chamber with an acoustic inertance passage and acoustic resistance means therebetween, the shape and dimensions of said inertance passage being such that the acoustic resistance thereof is sumciently small in proportion to the total acoustic resistance of the resonator so that the total acoustic resistance of said resonator is substantially independent of the length of said passage.

5. The structure of claim 4 in which the resonance chamber is connected with the inertance passage by one or more acoustic resistance openings of substantially negligible acoustic inertance.

6. In combination with a duct, a silencer comprising a resonator having a neck for attachment to said duct, said neck providing separate concentrated resistance means and inertance means for said resonator, one or more openings in said neck providing said concentrated acoustic resistance means of substantially negligible inertance and being of such shape and so located in the neck of said resonator that it is variable independently of the principal inertance means in the neck of said resonator, the shape and dimensions of said neck being such that the portion thereof not including said resistance means provides substantially all of the inertance of said resonator and has a resistance sufficiently small in proportion to the total resistance of said resonator that the total acoustic resistance of said resonator is substantially independent of the length of the inertance passage.

7. In a silencer adapted to be attached to a duct in which the sounds to be attenuated are borne, a resonator therein for attenuating sounds by means of acoustic impedance, said resonator being a structural combination comprising a chamber and an annular'passage with a wall therebetween and connected by one or. more openings, said annular passage being concentric with said duct, said chamber, passage and openings therebetween each respectively providing substantially all of the compliance, inertance and resistance forming said acoustic impedance.

8. In a silencer adapted to be attached to a duct in which the sounds to be attenuated are borne, a resonator therein for attenuating sounds by means of acoustic impedance, said resonator having a neck comprising the structural combination of an annular passage with openings in a wall thereof, said annular passage being concentric with the duct in which said sounds are borne, said passage and said communicating openings each respectively providing substantially all of the inertance and resistance of said neck.

9. In combination with a duct, a silencer comprising a resonator, said resonator comprising an acoustic compliance chamber, an annular acoustic inertance passage of substantially negligible acoustic resistance concentric with said duct, said passage connecting with said duct substantially throughout its intersection therewith, and one or more acoustic resistance openings between said annular passage and said chamber, said opening or openings having substantially negligible acoustic inertance.

10. The structure of claim 9 in which the acoustic inertance passage extends at substantially a right angle from the duct.

11. In combination with a duct, a silencer comprising a resonator, said resonator comprising an annular chamber surrounding at least a portion of said duct, an annular acoustic inertance passage of substantially negligible acoustic resistance concentric with said duct and surrounding at least a portion thereof, said passage connecting with said duct substantially throughout its intersection therewith, and one or more acoustic resistance openings between said e and said chamber. a

12 The structure of claim 11 in which the acoustic resistance openings are a series of spaced small holes.

13. In combination with a duct, 9. silencer comprising a resonator, said resonator comprising a chamber adjacent said duct, an acoustic inertance passage of substantially constant width extending radially from a perimeter of said duct and communicating with and extending from said duct substantially throughout the length of said perimeter, and one or more acoustic resistance openings between said radial inertance passage and said chamber.

14. In a silencer, a resonator comprising a side branch resonance chamber associated with a duct, an acoustic inertance passage communicating with and extending from said duct substantially throughout the length of the perimeter of said duct, and acoustic, resistance means comprising one or more openings and said chamber.

between said 6 15. In combination with a duct, 0. silencer comprising a resonator, said resonator comprising an acoustic compliance chamber, two adjacent annular disks spaced apart and mounted on said duct, the space between said disks opening into said duct substantially throughout a perimeter thereof, and one or more acoustic resistance openings between said compliance chamber and said space between said disks.

16. In combination with a through duct in which sounds to be attenuated are borne, a silencer comprising one of the chambers formed by a plurality of telescoped cylinders whereby an annular chamber is formed around a central cylindrical chamber, the other of said chambers forming said through duct, said first chamber forming a compliance chamber, an annular acoustic inertance passage of substantially negligible acoustic resistance opening into and being concentric with said duct, and one or more acoustic resistance openings between said compliance chamber and said annular acoustic inertance passage.

17. The silencer of claim 16 wherein the acoustic resistance openings are a series of spaced small holes.

18. The silencer of claim 16 wherein the acoustic resistance openings are a series of spaced small holes opening into that portion of the annular passage farthest removed from said duct, the total area of the openings being small whereby the tuning of the silencer is not sharp.

19. The method for tuning a resonator by means of acoustic inertance without substantially affecting the decrement thereof which comprises 00 connecting the resonance chamber of said resonator with the duct containing the sounds to be attenuated by means of a concentrated resistance element and a passage of substantially negligible resistance and varying a dimension of said passage.

20. The method for varying the decrement of a resonator without substantially affecting the resonance frequency thereof, which comprises connecting the resonance chamber of said resonator with the sound duct by means of a passage of negligible resistance, and one or more openings comprising concentrated resistance between said chamber and said passage, said opening or openings having substantially negligible acoustic inertance, and varying the concentrated acoustic resistance by changing the area of the opening or openings between said resonance chamber and said passage.

21. The construction of claim 14 in which a second resonance chamber is connected to said first resonance chamber by means of the combination of an acoustic inertance passage of substantially no acoustic resistance, and acousticresistance means comprising one or more openings between said second resonance chamber and said second inertance passage.

22. The structure of claim. 3 in which said chamber contains a sound absorbing material.

23. In combination with a duct, a silencer comprising a substantially closed shell surrounding at least a portion of said duct and enclosing a chamher, two adjacent, spaced-apart annular disks extending outwardly from said duct and within said chamber, the space between said disks opening into said duct, and one or more acousticresistance openings between said chamber and said passage between said disks.

WILLIAM A. JACK, 3:1).